This invention relates to a method and system of detecting when the tape is attached to the machine reel during the automatic tape threading process of a magnetic tape system. More paricularly, it relates to a method and system that has no moving parts and requires no adjustments.
Magnetic tape systems have a file reel and a machine reel. The file reel is removable and contains the magnetic tape that will be written on, or read from, by the magnetic tape system's read/write heads. The machine reel is nonremovable and acts as a storage reel for the magnetic tape from the file reel as it is read from or written on by the read/write heads.
The path that the magnetic tape takes between the file reel and the machine reel is usually quite serpentine, passing over and around a multiplicity of idler rollers and driver rollers. To relieve the operator of the task of threading the tape through the path every time the file reel is changed, systems that automatically thread the tape have been developed.
In a typical automatic threading system, the operator places the new file reel on the file spindle and presses the LOAD (or its equivalent) switch. The file reel turns slowly while air pressure is used to direct the tape along its path to the machine reel. Sometimes jets of positive air pressure blow the end of the tape in the desired direction and at other times a negative air pressure is used to draw the end of the tape to a desired location. The details of automatic tape threading are well known and documented in the prior art.
Some method of detecting when the tape is attached to the machine reel must be used with automatic tape threading systems so that the loading sequence can be stopped and the next step of the operation started. In the prior art, this is usually done with a pressure switch. Typically, the hub of the machine reel is hollow and has several holes drilled in its circumference. Air is drawn through these holes and through a pressure sensing switch. When the tape is directed to the hub of the machine reel, the air being drawn through the holes sucks the tape down on the hub. As the hub rotates, it winds the tape around the hub until it overlaps. This covers all the holes and the air pressure through the switch drops, causing the switch to actuate, signalling that the tape is attached.
Unfortunately, this type of attachment, though simple, has several disadvantages: (1) the switch, being mechanical, is subject to wear and therefore to eventual failure; (2) the switching pressure of the switch must be manually adjusted, in some cases at periodic intervals; (3) not every tape attachment will cause the same level of pressure drop, due to abnormalities in the tape surface and the manner in which the holes are covered, compounding the switch adjustment problem; and (4) changes in air pressure, e.g., leaks in the system, changes in the pump drawing the air through the holes, changes in atmospheric pressure, etc., can cause the switch to actuate at the wrong time.
As described above, it should be evident that the pressure sensing switch is an unreliable and unsatisfactory method of detecting tape attachment. It should also be evident that there exists in the art a need for replacing the pressure switch with a more reliable method of detecting tape attachment.